<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <meta http-equiv="X-UA-Compatible" content="ie=edge" />
    <title>Document</title>
    <style>
      body {
        margin: 0;
      }
      .triangleBox {
        height: 100vh;
        display: flex;
        justify-content: center;
        align-items: center;
      }
      .triangle {
        width: 0;
        height: 0;
        border-top: 100px solid transparent;
        border-right: 100px solid red;
        border-bottom: 100px solid transparent;
        border-left: 0px solid transparent;
      }
    </style>
  </head>

  <body>
    <div class="triangleBox"><div class="triangle"></div></div>
  </body>
</html>
